Speech garbling apparatus



TRANsMri' E. N. DINGLEY, JR

SPEECH GARBLING APPARATUS Filed Oct. 3, 1940' TRAN SMIT A RECEIVE EDWARD N. DlNGLEY,Jr-

INVENTOR ATTORNEY Patented Nov. 7, 1944 UNITED STATES PATENT OFFICE srnccn GARBLING APPARATUS Edward N. Dingley, Jr, Arlington, va; Application October 3, 1940, Serial No. 50,503 1 (c1. lie-1.5)

(Granted under the act of March 3, 1883, as amended April 30, 1928; 37.0 0. G. 757) My invention relates to improvements inap paratus for separating, inverting, and recombining certain portions of the electrical wave energy representing speech transmission over wires in apparatus for reversing the aforestatedprocess into acoustical so as to'convert the wave energy thus made unintelligible into intelligibl speech.

The major purpose of my invention is to provide means whereby voice conversations transmitted by wire or radio may be rendered unintelligible to the casual listener-in.

The foregoing and other objects of my invention will clearly appear by referenc to the following specifications and drawing in which the single figure is a schematic diagram of my novel apparatus.

Referring to the drawing, the microphone I, of conventional type converts acoustical speech energy into electrical energy in the primary of transformer 2 in the conventional manner. The secondary of the transformer 2 is connected through the switch 20 (in the upper or Transmit position) to the input terminals 3 of the highpass filter 5 and also to the input terminals 5 of the low-pass filter 6. The high-pass and lowpass filters 5 and '6 are of conventional design and are designed to hav a common cut-olf frequency such as 1000 cycles per second so that the output terminals 1 supply energy having fre quencies of 1000 c. p. s. and above to the resistor 9 and the output terminals 8 supply energy having frequencies of 1000 c. p. s. and below to the resistor l0.

The sum of the voltages existing across re sistors 9 and I is applied to the input circuit of the vacuum tube ll by connecting the grid I2 to the point l3 and by connecting the cathode l4 through the bias battery l to the point It. Th vacuum tube H may be a triode as shown or may be a tetrode or pentode as desired. Its sole purpose is to provide a high impedance coupling to the points l3 and I6 and to provide a certain amount of desired signal amplification.

From the manner in which the circuits are shown to be connected it is apparent that a signal having a frequency of say 750 cycles passing through the low-pass filter 6, will develop .a voltage between the points l3-I6 in opposite phase to the voltage it would have developed had it been able to filter 5.

From the foregoing, it is apparent that the signal present on grid l2 of vacuum tube H will consist of substantially all of the voice frequencies present in the secondary of transformer 2 and that the voice frequencies on the grid I2 in the range 1000 c. p. s. and below will be opposite in phase from these same frequencies as they pass only through the high-pass appear at the secondary of transformer 2.

The reversal of phase of substantially half of th voice frequencies renders the speech signal completely unintelligible. This unintelligible speech signal is delivered from the output circuit of vacuum tube H, through the coupling transformer 10, through the switch IS (in its upper or Transmit position) to the output terminals 20 and 2] where it may be transmitted over a wire line or used to modulate a radio transmitter.

The unintelligible voice signal, after transmission over a wire line or over a radio circuit, is received at a receiving point equipped with a duplicate of the speech. scrambling device shown. At the receiving point, the unintelligible signal is delivered to the input terminals 22 and 23 of the transformer 24. From the secondary of transformer 24, this signal is delivered through the switch 25 (in the down or Receive position) to the input terminals 3 of the high-pass filter 5 and to the input terminals 4 of the low-pass filter 6. The unintelligible speech is divided by the filters into frequencies above and below 1000 c. p. s. and combined, as before, in reverse phase in the resistors 9 and I0 so that the signal reaching grid [2 of vacuum tube ll contains substantially the same frequencies in the same phase relationship as the original intelligible voice signal at the secondary of transformer 2. This intelligible signal is taken from the plate circuit of vacuum :liube l I, through th output transformer l8, through the switch 19 (in its down or Receive position) to the electro-acoustical transducer or ear-Phones 25 where it is translated into an intelligible acoustical signal.

The cut-off point of the filters 5 and 6 should be as sharp as possible in order to reduce the range of frequencies which are transmitted with equal amplitude through both filters and which consequently completely cancel each other in the resistors :9 and I0. It has been demonstrated experimentally that as much as 20 percent of the entire speech spectrum may be cancelled out in this manner without appreciably reducing the intelligibility of normal speech.

It is obvious that the security and privacy of the above described system may be enhanced by occasionally varying, at pre-arranged times, the cut-off point of the filters 5 and 6 from 1000 c. p. s. to other frequencies near the center of the normal speech spectrum.

While the two functions of the network have been shown as being performed by switching from a transmitting instrumentality to a receiver, to avoid duplication of the principal part of the drawing, it will be understood that in practical. use it will probably be preferable to provide one network for transmission and another for rec'ep tion. It isalso to be understood that the term. garbling covers phase invertion of portions -'of' the same wave, whether changing from intelligible to unintelligible, or vice versa, since the same operations and apparatus are involved'in apparatus, comprising the other output terminal of each said filter to the center point of said impedance, a vacuum tube having at least an anode,.a cathode and a grid, said cathode and grid being respectively connected to opposite terminals of said impedance to apply potentials from said two filters to said tube in opposite phase, means to connect both said filtersselectively to said converting means or to receive a signal wherein upper and lower frequency portions are present in opposite phase, and output means connected between said anode I and saidcathode.

2. Speech garbling apparatus,- comprising means' for converting sound energy into corre- "spondingly'fluctuating electrical energy, a high pass and a low pass electric wave filter, the lower cut-off frequency of the former being substantially the same as the upper cut-off frequency of l the latter, means to supply said fluctuating energy to both said filters, an impedance, means connecting one output-terminal of each said filter to a respective terminal of said impedance, means connecting the other output terminalof each said filter to the center point of said impedance, a vacuum tube having at'least an anode, a cathode and a grid, said cathode and grid being respectively connected to opposite terminals of said impedance to apply potentials from said two cut-off frequency of the former being substan- .tially the same as the upper cut-off f-requencygof the latter, an impedance, means connecting one output terminal of each said filter to a respective terminal of said impedance, means connecting filters to said tube in opposite phase, and output means connected between said anode and said cathode.

' EDWARD N. DINGLEY, JR. 

